This invention relates to dot matrix printer systems, and more particularly to dot matrix printers employing a reciprocating hammer bank mechanism.
In a copending application for patent, Ser. No. 495,830, filed Aug. 8, 1974 for "PRINTER SYSTEM" by Gordon B. Barrus et al, assigned to the assignee of the present application, there is described an advantageous line printer for data processing applications utilizing a reciprocating hammer bank in which a plurality of individual hammers are mounted adjacent a printing line. Each of the hammer elements comprises a resilient magnetic strip which is normally retracted from the printing position by a substantially closed magnetic actuator circuit in which a common permanent magnet and a common return path are used. Generation of a compensating magnetic field by energization of a coil disposed adjacent the free end of the hammer in the magnetic circuit overcomes the magnetic bias and permits the hammer to fly forward to the impact point. This arrangement provides a compact and rugged high speed printer structure having particular ease of fabrication, assembly and maintenance.
When it is desired that a shuttling hammer bank of this type be operated in certain modes, substantial energizing loads are imposed on the energizing coils. As pointed out in the copending application, this type of printer has a capability for virtually arbitrary pattern generation, in that actuation at every possible printing position makes it possible to print an "all black" page or pattern, while graphs, bar charts and arbitrary characters can also be prepared. The magnetic elements of the described hammer bank are substantially enclosed by front, top and rear covers which provide a compact dust and particle protective enclosure. However, under high duty cycle operation there is substantial internal heat build up due to coil heating under high current loads. Excessive heat build up tends to introduce a slight but significant warping of the backplate, which in practice moves the hammer elements 5-6 mils away from the printing position, and causes a lightening of the dot pattern. Similarly, heating of the coil reduces the intensity of cancelling magneto-motive flux in the magnetic circuit, thus lowering the impact force of the hammer against the paper and resulting both in lightening of the dot pattern and an increase of the flight time of the hammer, introducing an imprecision in dot placement. These effects, individually and cumulatively, tend to limit some of the advantages otherwise afforded by this new dot matrix printing system. It is not economically desirable, however, to provide localized refrigeration within the substantially enclosed hammer bank assembly. Addition of high mass structures to the hammer bank assembly would adversely affect the high speed shuttle motion, and increasing the power capability of the energizing circuits would increase both mass and cost. It is therefore desirable to reduce the heating problems engendered by high power consumption without otherwise affecting the mechanical and magnetic operation of the hammer bank.